WALLA WALLA — The FBI is investigating the theft of a ceremonial tomahawk believed to be the weapon used to kill Dr. Marcus Whitman, a 19th-century missionary who was a leader of Northwest settlement.

Whitman and his wife, Narcissa, were killed by Cayuse warriors who blamed the Whitmans for a deadly measles epidemic. Tomahas, a Cayuse tribal member, is believed to have used the tomahawk to kill Marcus Whitman on Nov. 29, 1847.

The tomahawk disappeared from a display case during visiting hours at the Whitman Mission National Historic Site in southeastern Washington. The thief used a special tool to dismantle the case, said Roger Trick, chief of interpretation at the site.

"Someone brought in exactly the right-sized wrench," Trick said.

Steve Yu, a criminal investigator for the National Park Service, said there were no leads or suspects.

"It might be solved in a year; it might not show up for 20 years," Yu said.

The hatchetlike weapon is one of two "Whitman tomahawks" that may have been used to deliver the fatal blow. The second is on display at the Oregon Historical Society in Portland, where officials are careful to note there is no way to be sure if either one actually was used in the killing nearly 160 years ago.

Both tomahawks were fashioned of iron with hollow wooden handles and designed for dual use as hatchet-style weapons and ceremonial tobacco pipes.

Five Cayuse men were charged with killing 14 of 72 people at the Whitman Mission at Waiilatpu, "the place of rye grass," just west of Walla Walla.

Whitman, 45, was killed with a tomahawk; his wife, Narcissa, 38, died of gunshot wounds.

Historians think two of the men, Tomahas and Chief Tiloukaikt, probably took part in the slayings but that the other men may have surrendered to prevent destruction of the entire tribe by vengeful frontiersmen. The five men were tried in 1850 in Oregon City and hanged.

The man generally believed to have triggered the murders was Joe Lewis, from Maine, who arrived in Oregon in a wagon train, Trick said.

Lewis, half French-Canadian and half Delaware Indian, whipped the Cayuse into a frenzy over a number of deaths from measles, but he dropped out of sight after the attack on the Whitman Mission. Some think he was shot to death in a stagecoach holdup near Missoula, Mont., in 1862.

Transcripts of the Cayuse men's trial make it clear nobody knew what had happened to the tomahawk used in the killings, said Malissa Minthorn, archives and library manager at the Tamastslikt Cultural Institute on the Umatilla Indian Reservation near Pendleton, Ore.

Minthorn is a descendant of Kiamasumkin, or "Cougar Shirt," one of the men hanged. He claimed innocence to the very end, she said.

The tomahawk may have been nothing more than a prop that was featured in photographs taken by Maj. Lee Moorhouse of Pendleton, who produced 9,000 photos documenting Native American life between 1888 and 1916, Minthorn said.